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I was taking my old 1952 truck down to pick up my son at daycare/preschool today. As I waited to turn off of main street , smoke started billowing out from under the hood. I crossed the street onto a side street and shut the truck down and popped the hood. I found a wire spliced into the ignition circuit at the distributor, which fed some LED fog lights. When I bought the truck, there were halogen lights mounted on the bumper, and they were always on if the ignition was on. I replaced them with LED lights years ago but did not investigate the wiring. Last week I pulled the bumper to start painting and installing my LU-4 winch, and I did not cap off the wire. It had swung and contacted the frame and welded itself to it. I am assuming the insulation burned off relatively quickly. The wire was about 6-8' long, and all of the insulation was burned off of it. I have a few questions about this. I am a licensed electrician, and have a good grasp of the importance of proper fusing.
1) What is the wiring gauge of the wires feeding the distributor? The wire that burned was 12 to 14 gauge I am guessing

2) What is the fusing/breaker ampacity for the ignition circuit? With a dead short, the amp flow should have surpassed any breaker of fuse size available for these trucks. I am assuming the breaker did not function.

3) Besides a few frayed spots, the factory wiring looks ok. Only the added wire caught on fire, but I am leery of more hidden, damaged wires. Excessive current had to flow through through the whole ignition circuit from the battery to the splice.

4) Is anyone using modern fuses for the factory wiring harness, and not using the original breakers? I have a fuse block which I have been meaning to install for the circuits I have added to the truck. Currently they are protected by inline glass fuses. Where are you putting the fuses, if you are using them? My added fused now are in the battery compartment, but a larger cable to under the dash would be a cleaner insulation

Wow-glad you're ok after such an ordeal! I can't answer your harness questions but will say that I installed a Painless Performance harness in mine, complete with fuse block, that replaced my entire wiring system altogether and has given me no trouble since the install.

Perhaps some of our harness experts will chime in....
Gary

What kind of painless performance harness did you get? did you have it customized to match up with the original harness, or did you have to do some work yourself to make it all fit? I've made wiring harnesses before and it's a LOT of work if you are just starting with wires and components...

Hi ZG,

Here's a few answers for you;

1. There's a single wire feeding the distributor, 14g, and it does NOT run thru a circuit breaker.

2. The circuit breakers are rated for 15 amps, slow blow. They get power from the ignition switch and they protect the lighting, horn, and auxiliary power port circuits.....not the ignition.

3. Yes, but it depends where the splice was put.....the whole circuit is....battery to regulator to ignition switch to coil (distributor).

4. Can't answer this one.

Looks like you experienced first-hand why the electrical system on these trucks is so dangerous. When I got my first M I drove it around the pasture a few times, then took a good look under the hood and dash. The fire-waiting-to-happen from the deteriorated electrical wiring made me park it at that point and I'm not starting it again until I re-do all the wiring.

I'm glad you're ok, as well as your truck.

regards,
bob

I am looking at the generator system electrical schematic, and I see how the power feeds from the regulator to the ignition and the circuit breakers. Is there any reason why I can't remove the wire (#10), from the regulator, and re-power it from a fuse block off the batteries? I am looking to abandon any wiring on the affected circuit and re-feed it from a fused source. I don't have any idea how this would affect the regulator. I replaced the mechanical regulator years ago with a solid state one a few years ago. It looks like the wire that burned up was between the ignition switch and the distributor, and was one of the few wires with no over current protection

Hi ZG,

Well...........you could........but if you powered wire #10 from the battery, vice the regulator, i#10 is going to see unregulated power (duh). So, for instance, when you hit the starter the battery voltage will drop......and so will the voltage on wire #10. The regulator maintains a constant 24V on #10 regardless of load on the battery. You'd also see the voltage on #10 drop whenever you hook anything up to the slave receptacle, if you have one on your truck. If you're running a radio in your truck you definitely DON'T want wire #10 unregulated. Why not just hook up your lights, or anything else you want to power, from the third breaker....which powers the aux. power port on early trucks. I'm not sure what it powers, if anything, on later trucks. You could also put another breaker in series with the others, on wire #11, and power anything you want. Just thinking out loud.

regards,
bob

I am mostly concerned about a possibly damaged #10 wire in the cable assembly from the distributor to the regulator and I am looking for a way to avoid replacing it. I should probably get a new cable and replace any other exposed wiring under the dash which might be damaged. And put a fuse in line

Looks like you've gotten plenty of advice, but here's the requested info:
I used the Painless 10104 non-GM kit along with their light switch (for now). Everything is marked, wire by wire, with plenty of crimp connectors included. I added shrink-wrap to each one, and Power Braid to add extra protection. You can test the circuits using the 2 amp side of a battery charger to ensure everything is wired correctly w/o causing damage if something is incorrect. Cost a bit less than going with the Vintage Wiring kit. Also remember that I'm on 12 volts with an alternator so a civvy vs military harness didn't cause me any concern.
Gary

Just a couple of things from what I've read in this post...
Wire 10 is from the regulator to the feed side of the first of the breakers. It does not go to the distributor. Wire 11 now daisy chains to the feeds of the other breakers and the input to the ign switch. Wire 12 goes from the ign sw. to the dist. Wire 27 goes to the spider and feeds the gauges from the ign switch, also. To protect these circuits, use one of the unused breakers output for the wire 11 to the ign switch.

Hi Just,

You could do that, but looking at the wiring diagram you can see the priority is keeping the truck running, not safety. By that I mean the designers set up the wiring to make sure the most important thing was to keep the truck running. The breakers are there to ensure the other "nice to haves" such as lights, horn, heater, etc cannot stop the truck from running if they malfunction, hence the breakers used in those circuits. The breakers protect the circuit providing power to the distributor. Moving wire #11 so it runs THRU a breaker means the ability of the truck to keep running now depends on the reliability of the breaker. It also means your putting the instrument panel wiring, with the exception of the ammeter (if you have one) in that same circuit, so any problems with that wiring and you trip the breaker and stop the motor.

That all being said, our trucks no longer see combat, so if you're concerned about the condition of your wiring, or just don't like the distributor feed being unprotected, then running #11 thru a breaker will do the trick. Just remember that you changed the wiring if you need to troubleshoot why the engine won't start and keep a spare breaker in the glovebox.



regards,
bob

I was doing a quick look under the hood today, and one of the small wires leaving the voltage regulator has been taped over with electrical tape. Most likely by the fire department that had it before me. It looks pretty frayed and bare on the little bit I can see between the regulator and the firewall. I didn't look further into the cab wiring. I am wondering if the four cables that leave the voltage regulator can be individually replaced, or is the whole cable assembly (with threaded connector) replaced at one time? If the conductors are not molded in, I might replace the one frayed connector and add an inline fuse, if it is the wire than feeds the ignition circuit. I need to pull the connector apart, and see what wire it is which is damaged

Hi ZG,

Their are either 4 wires or 2 wires leaving the regulator, depending on when your truck was made. The earlier trucks had the 4 wire harness because 2 of the 4 wires (#8 & #9) went directly to the ammeter in the instrument cluster. They carried the full current of the electrical system, so you really don't want them touching anything metal. When they changed the ammeter to a voltmeter those 2 wires disappeared as well. The other 2 wires, #4 and #10 are the same in all M37s. #4 goes to the starter and #10 goes to the breaker that feeds the ignition switch and the other breakers. Sounds like you got a 4 wire harness, so check those ammeter wires!

I haven't looked at the regulator harness connector that close, but I don't think it's designed to be taken apart........but I could be wrong. An easier solution is to buy one of those 60 amp conversions kits that are on on Ebay all the time. They go for like $15 but have a new regulator harness included. You can simply modify that new harness to fit your needs. Seems all the vendors want like $95 for a regulator harness, so the 60 amp kit is a bargain. Just FYI.........I wouldn't buy a NOS harness. Reason I say that is I've picked up a few thinking NOS was the way to go.......they look all nice and pliable in the box, but the problem is once you put it on the truck and the air gets to it..........it breaks down pretty quickly. If you don't want to go with the harness in the 60 amp kit I'd give the guys at Vintage Wiring of Maine a call and just buy a newly made regulator harness. Just my 2 cents.....which is about what it's worth.



regards,
bob

Thanks Bob. I changed my ammeter to the volt meter soon after I got the truck, probably in 2010. I also swapped the mechanical voltage regulator for a transistor VR. I think I will look into getting a two wire cable. I didn't know that was an option.

I try to avoid any NOS parts with rubber in them. Although I did install my NOS fan belt last week. It had a tag saying it was cured in 1965. It is a year older than me!